I thought it was the Senator somehow having acquired a system override and using Fury's name to hide his involvement, but I'm not sure if that was clarified.

I hadn't thought of that! But actually that'd also fit well with the rumors that Chris Evans wants to stop acting (though I think he has one movie contract left after Cap 3? Oh well, flashbacks.) And then can we have a movie about Bucky-as-Captain-America and Sam Wilson/Falcon working together to a) find and capture Steve's killer, b) heal? I want it. Natasha can be in it, and Clint. And Carol Danvers. (And Eli Bradley because yolo.)